Yes, it should be explicitly mentioned, that deletion isn't guaranteed. But I don't think it's a deal breaker. People should be aware that posting something publicly on the internet, must always be considered to be preserved into infinity.
If you post something publicly on Facebook, there is also a high probability that some web scrapers save your pictures and posts. Deletion does nothing there.
Not women only but women centric and with additional moderation protection. And yeah, I'd also say that women should administer and moderate such an instance.
"Are there any Lemmy/Mbin instances by women for women?"
Where is the division?